Pentareflectors like pentaprisms redirect light so that the rays of an entering beam intersect corresponding exiting rays at right angles without requiring a critical or constant relationship of the pentareflector to the entering beam.
Pentaprisms have internal reflecting surfaces that are not only optionally flat, but both of those surfaces must be precisely 45.degree. to each other and at right angles to a common reference plane.
Pentareflectors commonly have two front-surface reflecting plates mounted on a base plate. Each reflecting plate must meet difficult combined standards, not only of optical flatness but also a critical right-angled relationship between its reflecting surface and its mounting surface. Where there is deviation from the combined standards, there is degradation of the exciting beam as compared to the entering beam or inaccurate redirection of the light, or both. Additionally, there is the further complication of bonding material, when used, producing distortion of the reflecting surfaces when there are defects in the mounting surfaces.
The user of pentaprisms and pentareflectors should have a way of recognizing the reference plane mentioned above, for properly mounting the device in an optical system. In the case of a pentaprism, yet an additional flat surface of the prism serves as the reference plane, both during manufacture and in later use. The lower surface of the base plate of pentareflectors may serve similarly as the reference plane.